Thin film capacitors are becoming increasingly important components in high frequency hybrid microelectronics and in advanced packaging arrangements such as multichip modules (MCMs). They are used in distributed RC networks and as decoupling capacitors to protect active devices from switching transients.
Thin film capacitors for advanced packaging applications advantageously exhibit a high capacitance density, low leakage current and a high breakdown voltage, all in large area thin films. In addition the capacitors should be compatible with subsequent steps in MCM processing. For example, the capacitors should possess sufficient thermal stability to undergo the thermal cycling up to 350.degree. C. used to cure polyimide layers in MCMs. The capacitors should be made from materials compatible with materials commonly used in MCMs.
Because of its excellent dielectric properties, Ta.sub.2 O.sub.5 has been extensively investigated as a dielectric for thin film capacitors, but these efforts have failed to find a method or structure that is optimal for advanced packaging. Efforts to make capacitors using Ta.sub.2 O.sub.5 films deposited by reactive sputtering, chemical vapor deposition (CVD), and plasma enhanced chemical vapor deposition have produced devices having high leakage currents and low breakdown voltages. See T Aoyama et al., "Leakage current mechanism of amorphous and polycrystalline Ta.sub.2 O.sub.5 films grown by chemical vapor deposition", J. Electrochem. Soc., Vol. 143, No. 3, pp. 977-983 (March 1996).
Capacitors fabricated with anodized reactively sputtered Ta.sub.2 O.sub.5 films were found to have superior leakage and breakdown properties. Unfortunately the films degraded upon thermal annealing above 200.degree. C., with irreversible increases in the temperature coefficient of capacitance (TCC) and the dissipation factor. See J. M. Schoen et al, "The correlation between temperature coefficient of capacitance and dielectric loss in tantalum and tantalum-aluminum anodic oxides," J. Electrochem. Soc., Vol. 119, pp. 1215-1217 (September 1972). This degradation is believed to be due to the diffusion of electrode metal atoms into the dielectric and diffusion of oxygen out, creating oxygen deficiency defects.
A variety of electrode metals have been tried to overcome this degradation problem, but thus far only tungsten was found to provide thermal stability. Chromium was tried but specifically rejected. See M. Peters et al., "Thermally stable thin film tantalum pentoxide capacitor", Proc. of the International Conference on Multichip Modules, Denver, pp. 94-99 (April 1996). Unfortunately tungsten is a refractory metal not fully compatible with conventional MCM fabrication techniques. Accordingly there is a need for a new method for making tantalum oxide thin film capacitors.